Conventionally, when driving insulated gate bipolar transistors (IGBT), which are voltage-driven semiconductor devices, it is known to provide an overcurrent protection circuit to protect IGBTs from overcurrent (for example, NPL 1). A common method of detecting overcurrent in such an overcurrent protection circuit is to use a current sense IGBT, shunting about one ten thousandth of the collector current of the main IGBT (sense current) to the sense IGBT and directing the sense current from the sense IGBT to a current detection resistor to compare the voltage obtained (sense voltage) with a reference voltage by a comparator.
The sense voltage is used to determine the magnitude of the current, on the basis of which a logic circuit causes an alarm to be outputted or the gate voltage to be shut down. FIGS. 10A to 10C illustrate a switching waveform when an IGBT is turned on. As illustrated in FIG. 10B, when at time t1 gate voltage Vg is applied to the gate terminal, the gate current charges the gate capacitance and the gate voltage starts to gradually increase. This leads to a relatively gradual decrease in the collector-emitter voltage Vce of the IGBT as illustrated in FIG. 10A. Then, when the gate voltage equals the on-voltage at time t2, the collector current Ic starts to flow. Subsequently, when the gate voltage Vg equals the Miller voltage Vm at time t3 and the Miller period starts, the collector current Ic starts to flow. At this time, the collector current Ic sharply increases and, after overshooting, shifts to a steady current state.
During the Miller period, fluctuations in the collector-emitter voltage Vce change the gate-collector capacitance Cgc and, to charge and discharge the gate-collector capacitance Cgc, the gate voltage is kept constant. The Miller period ends at time t4 after the collector-emitter voltage Vce decreases to 0V, and the gate voltage Vg starts to increase again and becomes constant at time t5 when it equals the power source voltage Vcc.
At this time, the gate current flowing between the gate and the emitter of the sense IGBT to charge the gate capacitance flows to the current detection resistor, causing transient sense voltage Vtr in the sense voltage Vs of the IGBT during the turn-on time, as illustrated in FIG. 10C. Also, during the turn-on time and the turn-off time, the gate voltage is lower than the power source voltage Vcc and the on-voltage i.e. on-resistance of the IGBT is larger, and consequently the ratio of resistance of the sense unit consisting of the on-resistance of the sense IGBT and the current detection resistor becomes relatively small, which results in increased sense current, leading to an occurrence of transient sense voltage Vtr as a function of the increased sense current.
Therefore, during the turn-on time the above-described two transient sense voltages Vtr are superimposed onto the sense voltage Vs and the sense voltage Vs thereby becomes higher than the overcurrent threshold voltage Vth. When the gate voltage Vg equals the power source voltage Vcc, the sense voltage Vs on which the transient sense voltages Vtr is superimposed becomes lower than the overcurrent threshold voltage Vth and then becomes constant.
Accordingly, during the turn-on time, the sense voltage Vs corresponding to the current outputted from the current sense terminal of the IGBT is in a superposition mode, in which transient sense voltage Vtr is superimposed, and then shifts to a normal mode, in which the transient sense voltage is not superimposed due to the gate voltage having reached the power source voltage. Also during the turn-off time, the sense voltage Vs is in the superposition mode, in which the transient sense voltage Vtr due to the fall of the gate voltage Vg is superimposed onto the sense voltage Vs.
In the superposition mode, the sense voltage exceeds the overcurrent threshold voltage as transient sense voltage is superimposed and, in this state, an overcurrent detection circuit would detect an overcurrent state, which would be a false detection. To prevent such a false detection, as illustrated in FIG. 10C, it is necessary to set a false overcurrent detection prevention period T1 (for example, 3 μs, during which the outputs from the overcurrent detection circuit are invalidated) that corresponds to the period during which the sense voltage exceeds the overcurrent threshold voltage as well as to set a detection period T2 of a certain duration (for example, 1 μs) after the false overcurrent detection prevention period T1, thereby setting a detection time T0 (for example, 4 μs), which is the sum of both periods.
To prevent such false detection of an overcurrent state, it has been proposed, as recited in PTL 1, to set the gate threshold voltage VGE(th) S of the sense IGBT cell forming the sensing circuit at a larger value than the gate threshold voltage VGE(th) M the main IGBT cell forming the main circuit to delay the rise of the main current in the sensing circuit with a time lag Δt during the turn-on period in comparison with the main current of the main circuit to prevent surge current from appearing in the sensing current. In this case, surge current is prevented from appearing also during the turn-off time.